Prostate cancer (PCA) has become the most commonly diagnosed life-threatening cancer in the United States. Epidemiological studies have suggested that environmental factors, particularly the diet, play a significant role in PCA pathogenesis. New molecular clues to the etiology and pathogenesis of prostate cancer are needed to account for the environmental influences on PCA development and to engender new rational approaches for prostate cancer prevention. Studies accomplished in the first three years of this project have revealed that somatic inactivation of the human gene encoding the pi-class glutathion S-transferase (GST), GSTP1, nearly uniformly accompanies human prostatic carcinogenesis. Most often, GSTP1 inactivation appears to be the result of "CpG island" DNA methylation changes were detected in DNA from more than 90 percent of PCAs and from some 70 percent of prostatic intraepithelial neoplasia (PIN) lesions, thought to represent PCA precursors. GSTs are known to detoxify oxidants and electrophiles that threaten genome damage. Data collected thus far support a new hypothesis for the pathogenesis of human prostate cancer: prostatic cells containing defective GSTP1 alleles may be vulnerable to neoplastic transformation and malignant profession triggered by exposure to oxidants and electrophiles. To continue to address this hypothesis, new specific aims are proposed for the next three years, including experiments designed: (1) to further characterize the nature and extent of somatic GSTP1 "CpG island" methylation changes in PCA and PCA precursor lesions, (2) to assess the expression of the maintenance DNA methyltransferase (DNMT1), (3) to characterize the effects of abnormal "CpG island" methylation changes on GSTP1 promoter function, and (4) to determine the phenotypic consequences of loss of GSTP1 expression on prostate cancer cell susceptibility to DNA damage mediated by prolonged oxidant stresses.